1. Field of the Invention
The present invention relates to a backlight device, and more particularly to a side-lit backlight device employing a point light source.
2. Description of the Related Art
In nonluminous display devices such as liquid crystal display devices, so called backlight devices that illuminate display panels from behind are generally provided. These backlight devices divide into a direct-lit type, a side-lit type, a planar light source type, etc., and the side-lit type is widely adopted these days with the trend for slimmer and lighter display devices.
In a side-lit backlight device, light from a light source is led into a light guide plate through a side surface thereof and is made to travel inside the light guide plate by total reflection; in addition, light is partly reflected on a reflective sheet fitted on a back surface of the light guide plate; thus light emerges through a main surface of the light guide plate to act as a planar light source, to thereby illuminate a back surface of a display panel.
As light sources, although cold cathode tube lamps, which are linear light sources, have conventionally been used, with increasing consideration to the environment these days, LEDs (light emitting diodes), which are point light sources, have come to be increasingly used.
The light emission efficiency of LED devices, however, is about 10% at present, meaning that the remaining 90% of the light they emit is dissipated as heat. On the other hand, it is known that the light emission efficiency of an LED device decreases as temperature rises. According to one account, as temperature increases by 1° C., light emission efficiency decreases by approximately 1%.
Accordingly, if the heat generated by an LED device is left undealt with, the heat generated by the LED device itself diminishes the light emission efficiency and shortens the lifetime. In particular, when an increased number of LEDs are mounted with a view to achieving high brightness in backlight devices, the total amount of heat generated by the LEDs is considerably large; thus making the above problem notable.
Thus, for example, a technology is proposed (e.g. Patent Documents 1 and 2) according to which, apart from a mounting conductor for an LED, a heat dissipation conductor is formed on a circuit board, and in addition, a container forming the LED is connected to the heat dissipation conductor via an insulating bonding agent to conduct the heat generated by the LED device to the heat dissipation conductor formed on the circuit board via the container and the bonding agent, so as to dissipate the heat therefrom into the air.    Patent Document 1: JP-A-2005-283852 Publication    Patent Document 2: JP-A-2006-11239 Publication